1. Field of the Invention
The present invention relates to a nuclear medical diagnostic apparatus such as a gamma camera, single-photon-emission computed tomography (SPECT) scanner or positron emission tomography (PET) scanner, and a composite medical diagnostic imaging apparatus such as a SPECT-CT scanner or PET-CT scanner combining SPECT/PET and X-ray CT (computed tomography).
2. Description of the Related Art
SPECT or PET obtains internal distribution of radioactive tracers given to a subject as a tomographic image. Gamma rays to be measured by SPECT/PET are decreased by Compton scattering and the photoelectric effect in the subject's body. Therefore, correction of the decrease of gamma rays is necessary for correct measurement of the internal distribution of radioactive tracers. This correction is called decrease correction. Decrease correction is a process of correcting a SPECT/PET image to correct the decrease of count value caused by decrease of gamma rays.
Decrease correction needs internal distribution of decrease coefficient of gamma rays. One method of obtaining the distribution of decrease coefficient of gamma rays uses a CT image obtained from the same measuring object. In this method using a CT image, the slice positions of CT and SPECT/PET images must be identical. To obtain the identical slice positions, a SPECT-CT apparatus or a PET-CT apparatus is used to obtain SPECT/PET and CT images. The other method uses different apparatus to obtain CT and XPECT/PET images, and adjusts the positions of the images by using software.
In both methods, the positions of internal organs are inevitably displaced in SPECT/PET and CT images due to different aspiration conditions. It takes long time to obtain a SPECT/PET image, and the position resolution is relatively low. Therefore, a SPECT/PET image is obtained in free aspiration. The time required to obtain a CT is short, and the position resolution is relatively high. Therefore, a CT image is obtained by stopping aspiration. According to the different aspiration conditions, the positions of internal organs are inevitably displaced in SPECT/PET and CT images, even if the same area of a subject is imaged. FIG. 6 shows changes in the positions of the liver according to different aspiration conditions.
The following methods have been taken to solve the above problem:    (1) Obtaining CT and SPECT/PET images in free respiration.    (2) Obtaining CT and SPECT/PET images by stopping respiration (see Jpn. Pat. Appln. KOKAI Publication No. 2005-195407).    (3) Imaging by acquisition in synchronization with respiration.
In all the above methods, CT and SPECT/PET images are obtained in the same respiration condition, and the positions of internal organs are not displaced in both images. However, in method 1, the quality of the CT image is degraded, and the quality of the CT image is unsatisfactory for diagnosis. A SPECT/CT image is complex in method 2, and the time required for imaging is long in method 3.